s390/vmemmap: fallback to PTEs if mapping large PMD fails

Let's fallback to single pages if short on huge pages. No need to stop
memory hotplug.

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200722094558.9828-8-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>